Chris R Stokes and Chris D Clarkf

*Landscape and Landform Research Group, Department of Geography, The University of Reading, Reading RG6 6AB, UK fDepartment of Geography, The University of Sheffield, Sheffield S10 2TN, UK

Ice streams not only exert a profound influence on ice-sheet mass balance and stability but they can also influence the ocean-climate system through profligate iceberg discharge and subsequent melting. In recent years, palaeo-ice stream imprints have been identified with a greater degree of confidence than ever before (see Stokes & Clark (2001) for a review) but further progress requires data to constrain ice-stream timing and their long-term evolution. One way to achieve this goal is to link the known location of palaeo-ice stream imprints (source) to the ocean sedimentary record (sink). Given an intact ice-stream imprint, it is possible to provide an estimate of the potential ice flux and the range of lithologies that should pass through the system, which can then be constrained and possibly dated by the ocean sedimentary record. Such a link has been made for the large Hudson Strait Ice Stream (Laurentide Ice Sheet) which is thought to have operated episodically throughout the last glacial cycle, contributing to Heinrich events and profoundly influencing North Atlantic ocean circulation and climate (Andrews & Tedesco, 1992). However, the extent to which other Laurentide ice streams influenced ocean circulation and climate is largely unknown.

One location where a large marine-terminating ice stream is known to have existed (at the very least during Late-glacial times)

is in the M'Clintock Channel at the northwestern margin of the Laurentide Ice Sheet (Hodgson, 1994; Clark & Stokes, 2001). The reconstructed location of the Late-glacial ice stream imprint is shown in Plate 26.1 with an example of the remarkable bedform imprint that it inscribed on Victoria Island (Plate 26.1b). The ice stream cut down through soft carbonate sediments and was comparable in size (ca. 770 x 140 km) to the Hudson Strait Ice Stream, with a surface area of ca. 162,000 km2, a catchment area of around 400,000km2 and an estimated cross-sectional area in excess of 100km2 (cf. Clark & Stokes, 2001). The final activity of the ice stream (ca. 10,400-10,00014Cyr) is constrained to around the same time as Heinrich Event-0 (Hodgson, 1994; Clark & Stokes, 2001) but whether it was part of a pan-ice-sheet response or acted independently is unknown. It is also unknown whether the ice stream operated at other times, prior to deglaciation, although the bedform record strongly supports an enlarged ice stream during full glacial conditions (Plate 26.1a).

More recently, marine sedimentary evidence has been documented which may shed some light on the long-term history of the M'Clintock Channel Ice Stream and its influence on Arctic oceanography and climate. Of major significance is that Arctic Ocean circulation was markedly different from present-day patterns (see Plate 26.2). During glacial intervals, Bischof & Darby

(1997) suggest that icebergs issued from the vicinity of Victoria and Banks Island are able to exit the Arctic Ocean directly through Fram Strait and down the east coast of Greenland, without the multiple rotations in the western Arctic Ocean which characterize present-day conditions (the Beaufort Gyre). The significance of this is that the transport and subsequent melting of icebergs from the Canadian Arctic Archipelago could have played a key role in reducing surface water salinity and North Atlantic Deep Water formation in the Greenland-Iceland-Norwegian Seas (cf. Bischof & Darby, 1997). Indeed, evidence from a sediment core taken from Fram Strait (Plate 26.2) suggests that there were four major iceberg discharge events between 12 and 31 ka (14Cyr) (Darby et al., 2002). These events were of relatively short duration (<1-4kyr) suggesting rapid purges of ice through Fram Strait. The composition of these iceberg rafted debris (IRD) events exactly matches a till sourced from Victoria Island, and we suggest that an enlarged M'Clintock Channel Ice Stream was the major contributor.

The far reaching effects of the ice stream are also evidenced in sediment cores from the Mendeleev Ridge in the western Arctic Ocean which also show peaks in carbonate IRD layers sourced from Victoria and Banks Island (Polyak et al., 2004) and major meltwater influxes (Poore et al., 1999). It is also interesting to note that, despite chronological uncertainties, the IRD events issued from the north-western margin of the LIS appear to immediately